The purpose of this study is to determine the effectiveness and safety of a recombinant (laboratory-produced) adenovirus injected into brain tumors. The virus has been especially designed to carry a gene called herpes simplex thymidine kinase (HSV-TK). The study will investigate whether this gene (a gene is part of the nucleus of a cell) will enter brain tumor cells and make them sensitive to the killing effect of the drug ganciclovir.